1. Field of the Invention
The present invention relates to use of an inhomogeneously broadened transition (IBT) material or spatial-spectral (S2) material to achieve optical analog signal processing, and in particular to processing analog, large dynamic range, high bandwidth, large time-bandwidth-product signals in the material.
2. Description of the Related Art
Real time analog large dynamic range signals with large time-bandwidth products (TBP, the product of a signal's bandwidth and its duration, typically considered large when in excess of 104) present a challenge for conventional digital processing. Such signals can arise in a variety of applications, including modern secure radio frequency detection and ranging (RADAR), light detection and ranging systems (LIDARs), Optical Time Domain Reflectometry (OTDR), spectral analysis and radio astronomy, among others. Other such signals are expected in a wide range of technical fields, including medical imaging devices, such as computer-assisted tomography (CAT) scans and nuclear magnetic resonance imaging (MRI), among others.
For example, the performance of arrayed RADAR systems depends strongly on the bandwidth and duration of signals the system can process in real time. Current RADAR systems are limited by available signal acquisition and processing technologies to bandwidths of a few hundred MegaHertz (MHz, 1 MHz=106 Hertz, Hz; a Hz is a cycle per second). Bandwidth directly translates into range resolution. Some applications, such as target recognition for missile defense systems, demand higher bandwidth signals in the range of multiple GigaHz (GHz, 1 GHz=109 Hz), at least a factor of ten greater than the available technologies. Coherent signal processing over multiple coherent acquisitions that are integrated means in RADAR to obtain the Doppler shift of a return signal, as well.
Based on the foregoing, there is a clear need for developing signal acquisition and processing technologies for signals with bandwidths greater than 1 GHz and TBPs greater than 104.
Advances in the growth and study of doped crystalline structures at extremely low temperatures (cryogenic temperatures), have revealed that the doping of certain rare earth ions in a certain way produces an inhomogeneously broadened transition (IBT) in these materials that display useful optical absorption properties. Materials that exhibit this IBT are called IBT materials. The absorption demonstrates optical frequency selectivity over bandwidths typically far greater than 1 GHz and with frequency resolution typically far less than 100 kiloHertz (kHz, 1 kHz=103 Hz). The ratio of bandwidth to resolution, which corresponds to a TBP, has been reported as high as 108 in some materials, and is generally considered useful when greater than 104. Therefore processors based on IBT materials have the potential to be able to provide signal acquisition and processing technologies for signals with bandwidths greater than 1 GHz and TBPs greater than 104.
The absorption features of ions or molecules doped into inorganic materials are spectrally broadened by two main classes of mechanisms. Homogeneous broadening is the fundamental broadening experienced by all ions or molecules independently, and arises from the quantum-mechanical relationship between the frequency lineshape and the dephasing time of the excited electron in the ion or molecule. Inhomogeneous broadening arises from the overlap of the quasi-continuum of individual spectra of all of the ions or molecules in the crystal, which have microscopically different environments and therefore slightly different transition frequencies.
The frequency selectivity can be modified locally by interaction with optical signals that excite electrons in the molecules that serve as absorbers from a ground state to an excited state, thereby removing those electrons and their host ions or molecules, at least temporarily, from the population of absorbers at that location in the material. Therefore, some such materials have been used to form highly frequency selective spatial-spectral gratings, and these materials are sometimes called spatial-spectral coherent holographic materials (S2 materials). After some time, the electrons may return to the ground state and the grating decays. When electrons are removed from the ground state in a particular homogeneously broadened absorption peak, a “hole” is said to be “burned” in the absorption of the material at that frequency, aid light at the frequency of the hole is transmitted with less absorption. Spectral gratings and spectral holes (features of spectral gratings) may be made permanent in some applications. Spectral grating and spectral holes are general terms that are interchangeable and herein are used interchangeably to describe aspects of a modified absorption profile.
Some IBT materials have been used as versatile optical coherent transient (OCT) processing devices. An OCT device is one with a broadband spectral grating in the optical range that extends over several homogeneous lines, and part or all of the available inhomogeneous broadening absorption profile. All the components of an optical spectral grating are typically formed simultaneously by recording the spatial-spectral interference of two or more optical pulses separated in time and/or space. When a grating is formed in the IBT material, the processor is said to be programmed, and the grating can be referred to as a device. This spatial-spectral grating has the ability to generate a broadband optical output signal that depends on an optical input probe waveform (referred to as a processing waveform) impinging on that grating and the programming pulses that formed the grating.
Various optical coherent transient (OCT) devices have been disclosed, such as an optical memory (for example, T. W. Mossberg, “Time-Domain Frequency-Selective Optical Data Storage,” Opt. Lett. 7,77 1982, and “Time domain data storage,” T. W. Mossberg, U.S. Pat. No. 4,459,682, Jul. 10, 1984), a swept carrier optical memory (for example, T. W. Mossberg, “Swept Carrier Time-Domain Optical Memory,” Opt. Lett. 17,535, 1992, and “Swept-carrier frequency selective optical memory and method,” T. W. Mossberg, U.S. Pat. No. 5,276,637, Jan. 4, 1994), an optical signal cross-correlator (for example, W. R. Babbitt and J. A. Bell, “Coherent Transient Continuous Optical Processor,” Appl. Opt. 33,1538, 1994, and W. R. Babbitt and J. A. Bell, “An optical signal processor for processing continuous signal data,” U.S. Pat. No. 5,239,548 Aug. 24, 1993), “Coherent time-domain data storage with spread-spectrum data pulse,” Yu Sheng Bai, Ravinder Kachru, U.S. Pat. No. 5,369,665, Nov. 29, 1994, “Optical cross-correlation and convolution apparatus,” Thomas W. Mossberg, Yu-Sheng Bai, William R. Babbitt, and Nils W. Carlson, U.S. Pat. No. 4,670,854 Jun. 2, 1987, “Reprogrammable matched optical filter and method of using same,” Xiao An Shen, Yu Sheng Bai, Eric M. Pearson, U.S. Pat. No. 5,381,362, Jan. 10, 1995), as an optical true-time delay regenerator (see for example, K. D. Merkel and W. R. Babbitt, “Optical Coherent Transient True-time Delay Regenerator,” Opt. Lett. 21, 1102, 1996), optical spatial router (for example, W. R. Babbitt and T. W. Mossberg, “Spatial Routing of Optical Beams Through Time-domain Spatial-spectral filtering,” Opt. Lett. 20, 910, 1995, and W. R. Babbitt and T. W. Mossberg, “Apparatus and methods for routing of optical beams via time-domain spatial-spectral filtering,” U.S. Pat. No. 5,812,318, Sep. 22, 1998), as means to continuously refresh and continuously use a spectral grating (published PCT patent application WO2000-38193 “Coherent Transient Continuously Programmed Continuous Processor”) and as a means to achieve processing and variable time delay (published PCT patent application WO2001-18818 “Method And Apparatus For Variable Time Delay Optical Coherent Transient Signal Processing.”) the entire contents of each of which are hereby incorporated by reference as if fully set forth herein, among others.
In these OCT devices, the readout approach typically uses high bandwidth optical signals, such as a coherent brief optical pulse or a series of coherent brief optical pulses, to probe the spectral grating, which under certain conditions can produce optical output signals that are generally referred to as stimulated photon echoes. A single brief coherent light pulse with the full spectral grating processing bandwidth stimulates a time-delayed output signal whose temporal profile represents the Fourier transform of the spectrum recorded in the grating structure. Thus a grating that represents the multiplication of the spectra of two programming pulses outputs a time-delayed signal whose temporal profile represents the convolution of those two pulses, e.g., the cross correlation profile of the two programming pulses. Probing with a series of brief coherent light pulses stimulates a photon echo signal whose temporal profile represents the correlation of the probe pulse with the Fourier transform of the spectrum recorded in the grating structure, creating real time processing ability of the probe pulse with a programmed spectral grating.
Frequency chirped pulses that are shorter in duration than the coherence decay time of the homogeneously broadened absorption lines have been used to probe spectral population gratings in inhomogeneously broadened absorbers. The result is a temporal output signal that is limited in duration by the coherence decay time. For example, chirped pulses can be used to store and recall temporal data (Y. S. Bai, W. R. Babbitt, and T. W. Mossberg, “Coherent Transient Optical Pulse Shape Storage/Recall Using Frequency-Swept Excitation Pulses,” Opt. Lett. 11, 724 (1986).) and used to generate arbitrary waveforms (Z. Barber, M. Tian, R. Reibel, and W. R. Babbitt, “Optical pulse shaping using optical coherent transients”, Opt. Exp. 10, 1145-1150 (2002)). The chirp rates of the chirped probe pulses used in these examples are fast and do not generate a temporal map of the structure of the spectral population grating. These chirped probes generate a temporal output signal that represents a collective readout of the spectral population grating, not its individual components, with bandwidths of the temporal output typically equal to the bandwidth that was excited in the medium, i.e. the bandwidth of the chirp. High bandwidth detectors are thus needed to record the outputs from this manner of chirped readout.
While potentially useful in many applications, the approach of readout creating an optical pulse or series of optical pulses at the full bandwidth of processing suffers, at present, from the limited performance in dynamic range of photo-detectors and digitizers that are needed to make a measurement of any high bandwidth optical signal. Existing high bandwidth detectors and analog to digital converters (ADC's also called “digitizers” herein) have limited performance (and are also far more expensive) as compared to lower bandwidth detectors and digitizers. For example, currently available detectors with bandwidths greater than 1 GHz have about 30 deciBels (dB) of dynamic range. Dynamic ranges of about 90 dB or more are preferred, which axe presently available for photo-detectors with bandwidths on the order of 1 MHz and ADCs with operation sample rates on the order of 10 Mega-samples per second (Ms/s).
In addition to the detection difficulties with high bandwidth signals, inefficiencies of such approaches of readout require that the high bandwidth probes have high power. Such high power probes are difficult or expensive to create, further impeding the implementation of these readout approaches.
Based on the foregoing, there is a clear need for a processor of high bandwidth signals that makes use of the TBP potential of IBT materials and that does not suffer the disadvantages of prior art approaches.